Two-dimensional fractal nanocrystals templating for SPECIAL FEATURE substantial performance enhancement of polyamide nanofiltration membrane Yang Lua, Ruoyu Wangb, Yuzhang Zhua,1, Zhenyi Wanga, Wangxi Fanga, Shihong Linb,1, and Jian Jina,c,d,1 aInternational Laboratory for Adaptive Bio-nanotechnology, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China; bDepartment of Civil and Environmental Engineering, Vanderbilt University, Nashville, TN 37235-1831; cCenter for Excellence in Nanoscience, Chinese Academy of Sciences, Beijing 100190, China; and dCollege of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China Edited by Howard A. Stone, Princeton University, Princeton, NJ, and approved December 29, 2020 (received for review September 24, 2020) In this study, we report the emergence of two-dimensional (2D) of the existing infrastructure or method of manufacturing TFC-PA branching fractal structures (BFS) in the nanoconfinement be- membranes and thus are prohibitively complex or too expensive tween the active and the support layer of a thin-film-composite to implement. A desirable approach for enhancing TFC-PA polyamide (TFC-PA) nanofiltration membrane. These BFS are crys- membrane performance should be simple, low cost, effective, and tal dendrites of NaCl formed when salts are either added to the readily integrated into the existing method of TFC-PA membrane piperazine solution during the interfacial polymerization process fabrication. or introduced to the nascently formed TFC-PA membrane before Herein, we report an elegant and highly practical method us- drying. The NaCl dosing concentration and the curing temperature ing two-dimensional (2D) fractal crystal dendrites to dramatically have an impact on the size of the BFS but not on the fractal di- increase the water permeance of the TFC-PA NF membrane ∼ mension ( 1.76). The BFS can be removed from the TFC-PA mem- while maintaining its solute rejection performance. By adding branes by simply dissolving the crystal dendrites in deionized NaCl to the aqueous PIP solution during the IP process, we water, and the resulting TFC-PA membranes have substantially observed that NaCl crystal dendrites emerged in the confinement higher water fluxes (three- to fourfold) without compromised sol- between the PA layer and the PES support when the TFC-PA CHEMISTRY ute rejection. The flux enhancement is believed to be attributable membrane was cured by heat (Fig. 1 A–C). These spectacular to the distributed reduction in physical binding between the PA branching fractal structures (BFS) are considered to be 2D be- active layer and the support layer, caused by the exertion of crys- cause they are less thick when compared with the overall size of tallization pressure when the BFS formed. This reduced physical the BFS sprawling along the plane parallel to the membrane binding leads to an increase in the effective area for water trans- port, which, in turn, results in higher water flux. The BFS-templating surface. Dissolving the PES support using dimethylformamide revealed a large number of crystals adhering to the bottom of the method, which includes the interesting characteristics of 2D crystal SI Appendix dendrites, represents a facile, low-cost, and highly practical method PA film ( , Figs. S1 and S2), confirming the position of of enhancing the performance of the TFC-PA nanofiltration mem- the BFS to be between the PA active layer and the PES support. brane without having to alter the existing infrastructure of Elemental analysis using energy-dispersive X-ray spectroscopy D E membrane fabrication. (Fig. 1 and ) and crystal structure analysis using X-ray nanofiltration membrane | high flux | interfacial polymerization | Significance fractal structure Fractal structures and phenomena have existed in nature for hin-film composite polyamide (TFC-PA) membranes are hundreds of millions of years. Developing their practical ap- Twidely used in reverse osmosis and nanofiltration (NF), plications in material design is of fundamental importance, but which have extensive and continuously growing applications in this goal has not yet been reached. In this work, NaCl crystals water treatment, desalination, and wastewater reuse (1–4). Typical with a fractal structure are formed between the polyamide TFC-PA membranes are fabricated using interfacial polymeriza- active layer and the support during an interfacial polymeriza- tion (IP), which involves a polymerizing reaction between tion process. The branching of fractal NaCl nanocrystals creates amine and acid chloride precursors at the water–oil interface numerous tiny interworking water channels that enable water (5–9). In a typical IP for producing TFC-PA NF membranes, a transport, maximizing the effective permeating area of the polyether sulfone (PES) ultrafiltration membrane is first impreg- polyamide nanofiltration (NF) membrane. The fractal NaCl – nated with an aqueous solution of piperazine (PIP) and then nanocrystals templated polyamide NF membrane exhibits an placed into contact with a hexane solution of trimesoyl chloride improved desalination performance with a three to four times (TMC). The PIP monomers diffuse across the water–hexane increase in permeance. Applying fractal structure successfully interface and react with the TMC to form a cross-linked dense to the design of artificial materials improves performance. PA film that serves as the active layer for water–salt separation Author contributions: Y.Z. and J.J. designed research; Y.L., R.W., and J.J. performed re- (3, 8, 10). This PA film is tightly bound to the underlying PES search; Y.L., R.W., Y.Z., Z.W., W.F., S.L., and J.J. analyzed data; and Y.L., Y.Z., S.L., and J.J. support layer, and the way they bind to each has a strong impact wrote the paper. on the water flux of the resulting TFC-PA membrane (11–15). The authors declare no competing interest. Enhancing the water flux of an NF membrane without com- This article is a PNAS Direct Submission. promising its solute rejection can potentially lead to substantial Published under the PNAS license. savings in treatment cost and has sizable practical impacts due to 1To whom correspondence may be addressed. Email: yzzhu2011@sinano.ac.cn, shihong. the broad application of TFC-PA membranes. While extensive lin@vanderbilt.edu, or jjin2009@sinano.ac.cn. research (9, 10, 16–25) has been performed with the goal of This article contains supporting information online at https://www.pnas.org/lookup/suppl/ performance enhancement, many promising approaches (10, doi:10.1073/pnas.2019891118/-/DCSupplemental. 16–25) reported in the literature require significant modifications Published September 7, 2021. PNAS 2021 Vol. 118 No. 37 e2019891118 https://doi.org/10.1073/pnas.2019891118 | 1of7 Downloaded by guest on September 26, 2021 Fig. 1. Formation process and surface morphology of BFS-templated TFC-PA membrane. (A) Schematic illustration of the process for preparing a BFS- templated TFC-PA membrane via interfacial polymerization. (B) Surface morphology of the BFS-templated TFC-PA membrane. (C) Close-up surface mor- phology of the BFS-templated TFC-PA membrane. (D and E) Elemental mapping images of Na (D) and Cl (E) on the surface of a BFS-templated TFC-PA − membrane. (NaCl concentration in PIP solution: 8 g·L 1; curing temperature: 60 °C). diffraction (SI Appendix,Fig.S3) confirmed that these 2D BFS be explained merely by the increase in the specific surface area, as were indeed NaCl crystals. the specific surface area of the BFS-templated TFC-PA mem- brane (after dissolving NaCl) was very similar to that of the con- Results ventional TFC-PA membrane (SI Appendix,Fig.S7). Instead, this The presence of NaCl crystal dendrites significantly increased flux enhancement is caused by the increase in the effective area of the surface roughness of the TFC-PA membrane (Fig. 2 A and the active layer for water transport (Fig. 2I). The PES support B). The thickness of the BFS is estimated to be 78 ± 10 nm based layer has a surface porosity of ∼15%, meaning that the PA active on an analysis of the surface topography measured using atomic layer is in direct contact with ∼85% of the underlying PES sup- force microscopy (AFM). While the NaCl crystals can be removed port. This fraction of the PA layer area either is not available or is simply by dissolving them in deionized water, the BFS formation ineffective for water and solute transport. has lasting impacts on the binding between the PA layer and the Such a theory is corroborated by the observation in previous underlying PES support. Unlike the tightly integrated interface studies (16–18, 20, 26) that a porous interlayer (between the between the support and active layers in conventional TFC-PA active and the support layer) with a very high porosity can sub- NF membranes (Fig. 2C), voids were created where the BFS stantially enhance the water flux. In the case of a BFS-templated existed before NaCl dissolution (Fig. 2D). TFC-PA membrane, the emergence and subsequent removal of Interestingly, the addition of NaCl and the formation of the the BFS reduced the area of direct adhesion between the PA BFS did not seem to affect the properties of the PA active layer active layer and the PES substrate, thereby increasing the effective itself. Both the reference TFC-PA membrane (without BFS area of the PA active layer for water transport (Fig. 2I). This formation) and the TFC-PA membrane (with BFS dissolved after argument is also well supported by an approximate quantitative formation) had practically indistinguishable properties, including analysis that investigated the porosity of the support layer zeta potential (Fig. 2E), water contact angle (SI Appendix,Fig. and the surface coverage of the NaCl dendrite (SI Appendix, S4), elemental composition (SI Appendix,Fig.S5andTable section 2.7). S1), distribution of functional groups (SI Appendix,Fig.S6), The morphology of the BFS depends on the NaCl concen- pore size distribution (Fig.